The modeling of High Speed Machining (HSM) spindles is a complex task due to the numerous physical phenomena involved in the dynamic behavior. Modeling is still rarely used in the industry, although sophisticated research work has been achieved. The boundary conditions of rotor models, which correspond to the ball bearings, are crucial and difficult to define. Indeed, they affect the dynamic behavior of the rotor in a non-linear and sometimes in an unpredictable way. The aim of the paper is to determine a relevant spindle model, i.e. the adequate level of complexity. To do so, a dynamic bearing model is introduced and the axial model of a spindle is established in relation to the preloaded bearing arrangement. Then, the operating stiffness of the spindle has been obtained experimentally with a new specific device that applies axial load and measures the resulting displacement, whatever the spindle speed. The model updating with the experimental data combined to sensibility analysis have led to the model refinement with additional physical phenomena, in order to account for non-linearities observed experimentally. The parameters of the model are also identified experimentally. As a result, a relevant spindle model is obtained and validated by the good agreement between simulations and experiments.